These are people who made lots of contributions:

Tatsuhiro Tsujikawa <tatsuhiro.t at gmail dot com>
Ross Smith II <aria2spam at netebb dot com> (Windows port)
Nils Maier <maierman at web dot Germany>

The aria2 contributor's list extracted from commit logs [1]:

Alexander Amanuel
Anthony Bryan
Athmane Madjoudj
Cristian Rodríguez
Dan Fandrich
David Macek
ITriskTI
Igor Khomyakov
Jarda Snajdr
Kcchouette
Nils Maier
ORiON-
Ross Smith II
Ryan Steinmetz
Ryo ONODERA
Sarim Khan
Sergey Zolotarev
Tatsuhiro Tsujikawa
Vasilij Schneidermann
Zoltan Toth-Czifra
amtlib-dot-dll
c3mb0
diadistis
gilberto dos santos alves
gt
kwkam
luokar
mozillazg
multisnow
oliviercommelarbre
Sonny Piers

[1] https://gist.github.com/tatsuhiro-t/deaffeb064652104ad11